Child Library updating from DMZ

I have a console and SUM in my DMZ which downloads directly from Sophos.

With EM Library I used to then share the Databank folder as a website so that the EM LIbrary within the LAN could dowload all the updates from the DMZ server rather than directly from Sophos.

Now I have put SUM on my LAN I can't work out how to set this up.  I just want my LAN based SUM to get its updates from my SUM in the DMZ via HTTP.  Which folder on the DMZ SUM should I share via IIS?  Or is there a better way of doing this now?

Thanks